This invention relates to arc-over reduction in cathode ray tubes.
Accelerating potential or ultor voltage for the electron beams within a cathode ray tube is generated by rectification of an alternating current voltage and coupling the rectified voltage to the ultor terminal of the cathode ray tube. The ultor terminal is coupled to an ultor inner conductive coating of the cathode ray tube envelope. The inner coating forms one electrode of an ultor capacitance. An ultor outer conductive coating forms the other electrode which serves as the ultor ground return terminal for ultor current.
The anode electrode of the electron gun is connected to the inner conductive coating, typically by means of spring contacts. The ultor accelerating potential serves as an energizing voltage for the anode electrode. Energizing voltages for the other electrodes, such as the screen, grid, and cathode electrodes, are developed by associated energizing circuitry external to the cathode ray tube. Connecting base pins in the base of the cathode ray tube couple the energizing voltages to the associated electron gun electrodes.
One of the connecting base pins serves as the ground return base pin for ultor current and is connected to the ultor outer conductive coating. Typical termination structure to the outer coating from the ground return base pin may comprise a U-shaped braid of copper wire closely pressed over the outer conductive coating on the cathode ray tube envelope. One or several copper wires connect various points along the copper braid to the ground return base pin. The ground return base pin is also connected to the common ground current return terminal of the energizing circuitry or to a chassis grounding terminal if the energizing circuitry and chassis have mutual ground current returns.
During cathode ray tube arc-over, the ultor voltage across the ultor capacitance may be coupled to several of the connecting base pins, thereby subjecting the energizing circuitry and other circuitry sharing common current return paths to overvoltages from the relatively low impedance ultor capacitance source. Sensitive semiconductor devices may be damaged due to the overvoltages developed and the relatively large arc-over currents flowing in the chassis and energizing circuitry.
Conventional techniques for arc-over protection involve shunting the arc-over current away from the energizing circuitry directly to the ultor ground return terminal through a spark gap coupled to the cathode ray tube connecting base pins. Relatively high impedance resistors may also be coupled between various ones of the base pins and the energizing circuitry to provide a voltage drop across the resistor for limiting arc-over voltages.
Nevertheless, arc-over current oscillation within the energizing circuitry may still develop. Stray capacity between the ultor ground return terminal and the chassis or energizing circuitry may provide a sufficiently low impedance AC current path from the ground return base pin to sustain relatively large and undesirable arc-over current oscillations. Such oscillations should be substantially reduced.